The invention relates to a method as well as a system for the transmission of cyclic and acyclic data over a transmission channel, taking into account real-time capability.
An industrial communications system, employed in data transmission systems, that connects a multitude of field devices such as measuring heads (sensors), final controlling elements and/or drives (actuators) to a control device is known as a field bus. A multitude of different field bus systems having different characteristics are established on the market, and are largely standardized worldwide.
Topologically, market-relevant field bus systems are currently subdivided, in essence, into two groups; on the one hand, into systems having a linear structure including strand structure or tree structure, such as Profibus, CAN or Ethernet, and on the other hand, into systems having a ring structure, such as Interbus.
Systems having a ring structure customarily are distinguished, in particular, by a high protocol efficiency, which is reflected in a high net data rate for a low transmission frequency. A ring topology implies a synchronous data transfer or data transmission of all users. Consequently, information to be transmitted is relayed from user to user until reaching the respective given user. Additional advantages are the excellent diagnostic characteristics with regard to the transmission link and the omission, to a large extent, of an allocation of device addresses and timings, since peer-to-peer connections are at issue for the transmission links, and consequently, unlike in a client/server or master/slave connection, a communication occurs among equals.
In contrast to this, the field bus systems for a topology based on a linear structure are, as a rule, hotplug-capable, i.e., it is possible to plug in and plug out individual users during running bus operation without much difficulty. In addition, process data and acyclic data, including device diagnosis data, can be transmitted, to nearly any extent, depending on the situation. These topologies are essentially based on a client/server or master/slave communication.
In general, however, it can be emphasized that the advantages connected with a ring structure correspond to the disadvantages of a linear structure and vice versa. In addition, particularly based on today's high complexity and high transmission speeds possessed by the presently established and accordingly market-relevant bus systems, a bus coupling frequently can be realized only by means of special and expensive protocol chips or μControllers.
One purpose of the invention is to identify a way of transmitting data by means of a transmission channel which effectively counteracts disadvantages previously resulting from a ring structure and linear structure and combines advantages of the one topology group with advantages of the other topology group, whereby to be ensured, in particular, are a hotplug capability and a situationally dependent transmission of process data and acyclic data, including diagnosis data, as well as a low protocol overhead and a synchronous data transfer and data transmission of process data to all connected users.
The solution according to the invention is given by means of a method according to Claim 1 and also by means of a transmission system according to Claim 20.
Advantages and/or preferred embodiments and improvements are the object of the respective subclaims.
According to the invention, it is therefore provided to improve a method for data transmission based on a protocol for cyclic data transmission by means of a transmission channel such that data to be transmitted in an impending cycle are checked against corresponding data of a preceding cycle for redundancy and currency, and in the event of recognition of an impending transmission cycle in which only redundant data without new information content is, in effect, to be transmitted, acyclic data is inserted, in lieu of the data without new information content, in the data field provided for said redundant data.
This preserves the equidistance of the sampling itself as well as the deterministic during the transmission of process data, which are particularly important criteria for control applications, without narrowing the user data throughput.
In order to carry out the method according to the invention, the invention proposes, in particular, a transmission system which in order to transmit and receive data is composed of a common data line between one master and at least one slave user. A multitude of linked slave users can, in this connection, form a user station to which the master is connected by means of the data line. The data transmission system is additionally composed of an application specific protocol, on the basis of which data is transmitted in cyclic time intervals between the master and slaves in order to write data in the slaves, and data is exchanged between the slaves and master in order to read out data from the slaves. The master possesses a dedicated test entity to enable checking data provided for an impending cyclic transmission with regard to its redundancy and currency in comparison to data, stored in a memory device, already transmitted during a preceding cyclic transmission, and also a deciding entity, coupled with the test entity, to enable deciding whether—and if yes, which—acyclic data is to be inserted in a data field in which for all intents and purposes only redundant data would be transmitted without new information content.
Advantageously, within each transmission cycle the master device reads out process input data from slave devices during a respective first data cycle, and during a respective subsequent second data cycle transmits process output data or the acyclic data.
In particular, it is provided that acyclic data can be inserted not only for transmission from the master device to slave devices, but also for transmission from slave devices to the master device.
In addition, a preferred improvement provides for cyclic data to be inserted not only for transmission from the master device to slave devices, but also for transmission between individual slave devices and/or from slave devices to the master device.
For this, it is provided that all user devices read information being transmitted at the same time during a data cycle.
For prompt error recognition, it is additionally provided that the user devices form a test sum or check sum by means of respective received or passively-monitored data, and compare this to a test sum or check sum appended to the received or passively-monitored data.
In a preferred improvement it consequently is provided for an error message to be transmitted back upon detection of an error.
In a practical manner, the invention provides for each data transmission during one data cycle to be commenced with the transmission of an address followed by a function code. A CRC generated by the master advantageously forms the closing.
The invention provides, in particular, for inquiry data, response data, or diagnosis data to be transmitted as acyclic data.
It moreover is advantageous if the master device predetermines a respective instant at which one or several slave users are to transmit acyclic data.
For this it is advantageous if, in order to predetermine such respective instants, the master device transmits, during a preceding data cycle within which acyclic data are transmitted, a corresponding function code at the same time.
In order to increase the error protection, in particular for security-critical processes, it is in addition provided that a normal data cycle comprising cyclic data be carried out at least after the transmission of a predetermined maximum number of consecutive data cycles comprising acyclic data.
Alternatively or additionally it is further provided in a preferred manner that if no error-free cyclic data exchange is activated within a predetermined period, an error is recognized and a pre-parameterized error process commenced.
For the transmission of data during each data cycle, the invention advantageously utilizes the constant external structure of a telegram assembled of an address, function code, a given number of user data and a test sum or check sum, with an Inter-Frame-Timeout, within which no data are transmitted, being advantageously provided between two consecutive telegrams.
The invention additionally proposes not performing a repetition of data transmitted in error during a data cycle until a subsequent corresponding data cycle.
In the event of recognition of the non-availability of a slave device during a data cycle of a contemplated data transmission, a particularly advantageous improvement of the invention further provides for the master device to embed a place holder in the corresponding time window assigned to the slave device.
During a data cycle, diagnosis data to be transmitted for evaluation are preferably commenced by means of a transmission request of the master device, whereby during such a diagnosis cycle information of higher priority is transmitted preferentially.
These and additional features as well as accompanying advantages of the invention are yielded from the following description of preferred exemplary embodiments, with reference being made to the enclosed drawings.
In the drawings,
FIG. 1 shows possible bus accesses within bus cycles preferably used for the method according to the invention,
FIG. 2 shows an example of a frame configuration for a telegram used according to the invention
FIG. 3 shows a telegram configuration during a PD-Write access cycle,
FIG. 4 shows a telegram configuration during a PD-Read access cycle.
FIG. 5 shows a telegram configuration during an acyclic embedded diagnosis access,